Field of the Invention
The present disclosure relates in general to Seebeck/Peltier thermoelectric conversion devices and in particular to devices utilizing parallel nanowires of conductor or semiconductor material extending across an insulating septum.
General Notions
The Seebeck effect is a thermoelectric phenomenon whereby in elongated conductors or semiconductors, a temperature difference between portions thereof generates electricity. The effect, discovered by the physicist Thomas J. Seebeck in 1821, manifests itself as a difference in potential between the ends of a metal bar subjected to a temperature gradient ∇T. In a circuit including two junctions between two materials A and B at temperatures T1 and T2, the resulting voltage is given by:
                    V        =                              ∫                          T              1                                      T              2                                ⁢                                    [                                                                    S                    B                                    ⁡                                      (                    T                    )                                                  -                                                      S                    A                                    ⁡                                      (                    T                    )                                                              ]                        ⁢                          ⅆ              T                                                          (        1        )            wherein: SA and SB are the Seebeck coefficients (also called thermoelectric power) related to two materials A and B. The voltage values are typically in the order of several μV/K. The Seebeck coefficient depends from the material, from absolute temperature and from morphological characteristics. The Seebeck effect can be exploited for measuring temperature in terms of voltage differences generated in a circuit comprising a junction of different materials (thermocouple) or for generating electrical energy by connecting in series a plurality of thermocouples (thermopile).
From a microscopic point of view, the charge carriers (electrons in metals, electrons and holes in semiconductors, ions in ionic conductors) will diffuse when one end of an elongated conductor is at a different temperature of the other end. The hotter carriers will diffuse towards the portion at lower temperature as far as causing different carrier densities in the cooler and hotter portions of the conductor. In an insulated system, equilibrium will be reached when by diffusion, beat will become uniformly distributed along the whole conductor. Redistribution of thermal energy due to the motion of charge carriers (thermal current) is obviously associated to an electric current which will nullify itself when the conductor temperature becomes uniform.
In a system wherein two junctions connected in a circuit are kept at a constant temperature difference, the thermal current will be constant too and therefore there will be a continuous flow of charge carriers in the circuit.
Mobility of the carriers is reduced by scattering phenomena at impurities, grain boundaries and similar defects present in the conducting material and by lattice vibrations (phonons). Therefore, the Seebeck effect is strongly dependent from density of impurities and crystallographic defects and from the phonon spectrum of the material.
Phonons move along the thermal gradient loosing energy by interacting with electrons or other carriers and with lattice imperfections, it is useful to define a thermoelectric factor of merit of a material as: Z=S2σ/κ, where κ and σ are the thermal and electrical conductivities of the material, respectively.
From the technological point of view, the use of generators based upon the Seebeck effect is considered potentially interesting. More than half the heat produced in a thermal plant is usually dissipated as low enthalpy heat, it is estimated that about 15 millions of megawatts are wasted in the energy conversion processes. Availability of Seebeck generators, able to convert even only partially such a waste heat in electricity would be able to impact positively on the energy problem.
However, thermoelectric generators have an extremely low efficiency. For example, in case of a thin film of n silicon, doped with 5×1015 As atoms per cm2, the factor of merit, at room temperature, would be Z≈3×10−5 K−1; whereas values of ZT≈1 can be obtained only with costly materials of limited availability such as Bi2Te3, Sb, Se and alloys thereof.
Practically, apart from some uses of high added value such as thermoelectric generation in space crafts, thermoelectric generators made with massive amounts of low cost materials of ample availability would achieve conversion yields of just about 7%. By comparison a turbine engine is able to convert about 20% of thermal energy in useful electrical current.
Recently it has been demonstrated [1, 2] that with a system based on an extremely slender conductor in the form of a silicon nanowire of about 20 nm and rugged outer surface, a high thermoelectric figure of merit can be achieved. The increased Z figure of the material derives from a decoupling of the free average paths of phonons and electrons caused by a different incidence of scattering at the surface of the two species. In particular, the important contribution to heat conductivity of acoustic phonons of lower frequency (of larger wavelength) is eliminated because the density of phonons of wavelength larger than the cross sectional size of the wire practically becomes null. Consequently, the heat conductivity of silicon drops from ≈150 W m−1 K−1 (at room temperature for massive Si) to 1.6 Wm−1 K−1 (at room temperature for 20 nm Si nanowires), whilst by contrast the electrical conductivity does not suffers an equivalent drastic decrement.
Formation of test nanowires of a suitable conductor or semiconductor material has for a long time been possible only with laboratory techniques hardly suited for fabricating thermo-electric conversion elements of commercially viable structure and size which could be associated for realizing septa of thermoelectric conversion, capable of functioning at commercially significative power levels and which could be industrially fabricated on a mass production scale.
A method of fabricating nanowires of elements of the IV Group of the Periodic Table or alloy thereof, without requiring advanced lithographic techniques, and including treatment steps for enhancing surface ruggedness, is disclosed in prior Italian patent application of one same applicant, filed on Apr. 11, 2008, published as WO 2009/12531. The method contemplates optional ion implantation and a thermo-cycling of the nanowires to induce creation of voids in the bulk of the material in order to usefully modify in a significantly different manner the mean free paths of phonons and electrons.
The prior application discloses the way a single level array of parallel nanowires and resultant structure may be fabricated in an industrially scalable manner, to be eventually stacked with similar single level array strictures in order to incrementally increase the size of the opposite surfaces, respectively hot and cold, of a septum of thermoelectric conversion, reduce internal electrical resistance and increase the power that can be yielded from the device.
The formation of a single level array of parallel nanowires on the surface of an insulating substrate by photolithographic definition, deposition of a conformal layer of conductor or semiconductor material and successive anisotropic etch of the conformal layer, requires the fabrication of innumerable of such “monolayer” elements, each through a repetition of the same sequence of photolithographic definition, deposition and etch, which makes the process relatively costly and limits the numerosity of nanowires that can be packed per unit of area of thermal input/output of a practical conversion device thus constituted.